Protective device



Feb. 6, 1945. a CAMILL', 2,369,015

PROTECTIVE DEVICE Filed Oct. 27, 1945 2 Sheets-Sheet 1 i 34 2e 27 50 & 33 30 l5 /4 E 42 A? as 85 2 /2 40 2o 2 3/ is, [8 24 26 Inventor;

/0 Guglielmo Camilli, a "l": U8 H by His Att orw'weyn Feb. 6, 1945. G. CAMILLI PROTECTIVE DEVICE Filed Oct. 27, 1943 2 Sheets-Sheet 2 m M 0 e Pm n w w n m m A h .5 I H (Bug! b Wan 17 an a e g I tion Serial No. 411,119,

Patented Feb. 6, 1945 PROTECTIVE DEVICE Guglielmo Camilli,

Pittsfield, Mass, asslgnor to General Electric Company, a corporation of New York Application October 27, 1943, Serial No. 507,849

20 Claims.

This is a continuation-in-part of my applicafiled September 17, 1941 My invention relates to protective devices, and although not limited thereto it has particular application to protective devices for preventing excessive voltages from being induced in the secondary windings of current transformers upon the secondary circuit being opened.

It has been the practice to provide some arrangement for short circuiting the secondary winding of a current transformer upon the secondary circuit being accidentally opened, since to allow the secondary circuit to be opened with the primary winding connected in a line, will cause very high voltage peaks to be induced in the secondary winding. Such high voltage in the secondary winding may not only injure the meters which are connected thereto but are dangercus to the operator of the meters. An example of a protective device which is designed to prevent such excessive voltages from being induced in the secondary winding of a current transformer upon the secondary winding circuit becoming accidentally opened, is my Patent 2,248,608, issued July 8, 1941, and which is assigned to the same assignee as this present invention. It is also desirable that the protective device be so constructed that there will be no exposed conducting part which may attain any appreciable voltage.

It is therefore an object of my invention to provide a protective device simple in construction and easy to manufacture,

1-3, as employed in the circuit of a secondary winding of a current transformer; Fig. 5 is an end view in partial section of a modification of the protective device illustrated in Figs. 1 to 4; Fig. 6 is a sectional view taken on line 6-5 of Fig. 5; Fig. 7 is an exploded perspective view of the parts of the protective device of Figs. 5 and 6; Fig. 8 diagrammatically illustrates the protective device of Figs. 5 to '7, as employed in the circuit of a secondary winding of a current transformer; Fig. 9 is a sectional view taken on line 9-9 of Fig. 5.

In the arrangement illustrated in the drawings, I have provided a protective device including at least a pair of electrodes and a resistance material' separating or between the electrodes, the latter being adapted to be connected across an electrical apparatus which is to be protected, such as the secondary winding of a current transformer. It is to be understood, however,

, that my protective device has application to any other apparatus which should be protected from abnormal high voltages. The protective device is a relatively high impedance for normal voltages in the secondary winding or other electrical apparatus to which the protective device is connected, but it provides a relatively low impedance and will short circuit the secondary winding upon a predetermined abnormal voltage being induced in the secondary winding.

. The resistance material is susceptible of becomwhich is not only but which will have no exposed conducting parts susceptible of attaining a voltage which may injure anyone who may come in contact with such conducting parts.

Another object of my invention is to provide an improved protective device of the type illustrated in my above mentioned patent.

A further object of my invention is to provide a protective device which will be simple in construction, eflicient in operation and have a relatively long life.

Further objects and advantages of my invention will become apparent from the following description referring to the accompanying drawings, and the features of novelty which characterize my invention will be pointed out with particularity in claims annexed to and forming a part of this specification.

In the drawings, Fig. 1 is an end view in partial section of a protective device'which is provided with an embodiment of my invention; Fig.

2 is a sectional view taken on line 2-2 of Fig. 1;

Fig. 3 is a sectional view taken on line 33 of Fig. 2 of a portion of the protective device illustrated in Figs. 1 and 2, showing the construction of a thermostatically operated switch which is employed therein; Fig. 4 diagrammatically illustrates the protective device illustrated in Figs.

ing overheated upon a current flowing therethrough, and so I provide an improved construction, including a thermostatically operated switch for short circuiting the electrodes and I therefore theresistance material, when the latter becomes overheated. One of the electrodes, in the construction shown in Figs. 1 to 4, is provided with a recessed portion providing a chamber facing the resistance material, for accommodating the thermal responsive device so that the latter may be responsive to heat from the resistance material. The other electrode is in the form of a cup shaped member so as to substantially enclose the first mentioned electrode I further find it extremely desirable to provide an arrangement for grounding the cup shaped electrode, the latter being susceptible of being touched by an operator, so that the operator will not be injured if he accidentally touches this exposed conducting part.

Referring more particularly to Figs. 1 and 2 of the drawings, I have rovided a protective device which includes an inner or first electrode I0 and an outer second electrode ll. As is seen more clearly in Fig. 2, the pair of electrodes I II and II are provided with faces which have substantially parallel adjacent portions I2 and I3 respectively. A resistance material I 4 separates is held between the'substantially parallel face portions l2 and ll of the electrodes. This resistance material may take any suitable shape, and it will be seen from Fig. 2, that a plate shape is a. convenient form, and the thickness thereof may be of any suitable value, depending upon the characteristics which are desired and the operating voltage and current of the device which is to be protected. Furthermore, the resistance material l4 may be composed of any suitable material, such as that having a non-linear resistance-ampere characteristic. I have found that a suitable material having such non-linear characteristics is that material having a hyperbolic resistance-current characteristic of the type described in Patent 1,822,742 McEachron, this patent being assigned to the same assignee as this present invention.

In order to provide an improved arrangement for short circuiting the resistance material l4 so as to prevent, it from becoming overheated when it operates to short circuit the secondary winding of the current transformer or other electrical apparatus which is being protected, 1 provide an improved structure which includes a suitable thermal responsive device such as a bimetallic element i5 and a suitable switch having contacts I6 and I! which are adapted to be actuated by the temperature responsive device l5, upon the latter being heated in any suitable manner as a function of the temperature of the resistance material such as by heat from the resistance material l4. The switch which is adapted to be actuated by the temperature responsive device may be of any suitable construction and in the arrangement illustrated, as will be seen clearly in Fig. 3, the contact i5 is mechanically carried by the end of the bimetallic element l5 and electrically connected thereto and the contact 11 is mechanically supported by and electrically insulated from the electrode III by an arrangement having a conducting portion and an insulating portion. This latter arrangement includes a conducting member H! which is insulated from the electrode ill by insulating washers l9 and Hi. In order to control the temperature at which the temperature responsive device will cause the switch contacts l8 and H to open and close I provide an ad Justment arrangement including a screw 20, which has an end 2! connected to the bimetallic element IS. The end 21 is insulated from the bimetallic element i5 by insulating washers 22 and it will be seen that when a nut 23 which cooperates with the bolt 20 and a face of the electrode I isloosened, the bolt 20 may be rotated, thus varying the distance betwee the contacts l6 and I1 and thereby controlling the distance through which the end of the bimetallic element 15 must traverse before it may open and close the contacts l6 and H.

In order to not only provide a convenient supporting arrangement for the bimetallic element l but so that it may be thermally associated with the resistance material, the electrode in is provided, as will be seen in Figs. 2 and 3, with a recessed portion 24 of a suitable shape so that it may accommodate the temperature responsive device and switch contacts. The recessed portion 24 may be formed in any suitable manner such as pressing out the portion 24 when the electrode in is being fabricated. It will there fore be seen that the recessed portion 24 forms a chamber 25 which opens toward the resistance material l4 so that the heat may pass quickly from the resistance material to the bimetallic element l5 within the chamber 25. It will fur ther be seen that a shoulder portion 26 is provided relatively close to the resistance material l4 upon which an end of the bimetallic element 15 may be supported so that this element may be supported relatively close to the resistance material I4 and thereby further improve the heat transfer characteristics between the resistance material l4 and the bimetallic element l5. It is to be understood, of course, that the recessed portion 24 may take any other suitable shape so as to accommodate the bimetallic element in order that it may be thermally associated with the resistance material i4.

In order to provide a convenient arrangement for electrically connecting .the electrodes in and I l to the secondary winding of the current transformer or other electrical apparatus which is to be protected, I provide terminals 21 and 28, which are connected to the electrodes in and H respectively. The terminal 21 has a flared end portion 21' which may be connected to the terminal In in any suitable manner, such as by brazing. Suitable nuts 29 are provided on the terminal 21 for tightly securing a connector 30 of a conductor 3| to the terminal 21. The terminal 28 is similarly provided with suitable nuts 32 for connecting a terminal 33 of a conductor 34 to the terminal 28. It will be further seen in Fig. 2 that the electrode Ill and resistance material l4 are provided with holes or bores 35 and 36 in substantial registry for accommodating a portion of the terminal 28. An end 31 of the terminal 28 may be electrically connected to the electrode i3 in any suitable manner, such as by brazing, the end 31 being electrically insulated from the resistance material 14 and surfaces of the bores 35 and 36 by an insulating ring 33. The terminal 28 is also insulated from the terminal ill by a portion of the ring and by the air space between the surface of the bore 35 and the terminals 28.

Since the thermostatically operated switch with contacts l6 and H are, upon closure thereof, to short circuit the electrodes in and II, an improved arrangement is provided for electrically connecting the contacts i5 and H to the electrodes in and II, respectively. The contact H5 is electrically connected to the electrode llJ through the bimetallic element l 5 and a rivet .39, which also provides for the mechanical supporting arrangement for the bimetallic element 15. The contact i1 is connected to the electrode ll through the rivet i8, which provides the supporting arrangement for the contact 11. This rivet is in turn connected to the terminal 28 by a conducting strap 40, which, as will be seen in the drawings, has one end connected to the rivet i8 and an opposite end 42 connected to the terminal 28. The latter connection is made through metal washers 43 and 44 and the adjacent nut 32. The metal washer 43 is in turn insulated from the electrode in by an insulating washer 45.

Operation of my protective device is as follows: As has already been stated, my protective device may be connected across the terminals of any suitable device such as the secondary winding of a current transformer in order to prevent an abnormal voltage from being induced therein. Referring to Fig. 4, it will be seen that there is schematically illustrated a current transformer having a primary winding 46, which is adapted to be connected in a line and a secondary winding '41, which is connected to the terminals 2"! and 28 through the conductors 3| and 34. There is also circuited. Such is very liable to happen since the meters are removably connected to the secondary winding 41; as it'may be necessary to connect and remove the meter several times during a metering operation. It will be understood that during a normal voltage being induced in the secondary winding, that substantially no current will flow through the protective device, as the resistance material l4 has such characteristics that when a predetermined low voltage is impressed across it, it will be substantially an insulator. The characteristics of the material l4 may be so designed, however, that upon a predetermined abnormal voltage being impressed across the terminals 21, 28, that is, when the secondary winding circuit is opened. the resistance of the material l4 will be substantially lowered thereby forming a relatively low resistance path for the current through the protective device. It has been found, however, that it is necessary to in turn short circuit the resistance material I4 under these conditions .to prevent it from becoming overheated and low resistance path for the current flowing in the secondary winding circuit, sothat high voltage will also be prevented from being induced in the secondary winding 41. The temperature responsive device I5 may be designed to have such characteristics that it will in turn open its contacts l6 and I1 upon the temperature of the resistance material I4 having dropped to a predetermined value. The protective device may therefore operate in this manner until the operator notices the open circuit and rectifies the difficulty.

It will be seen from Fig. 4 that the line 34, which connects through the terminal 28 to the electrode II is grounded at 49, so that the electrode II will be maintained at substantially ground potential at all times. Since the electrode ll substantially encloses the electrode Ill and resistance material I4, the chances of the operator, upon accidentally touching the protective device coming in contact with conducting mate- 5 rial which may be at high voltage, is reduced to a minimum. Furthermore, an insulating support may be made of any suitable shape and size so as to prevent the operator from touching the end of the terminal 21. It will be apparent that the protective device may be supported on a base through the supporting member 50 so that the electrode Ill will be adjacent the base and thereby reduce to a minimum the possibility of operator accidentally touching the electrode Ill. The support 50 is shown as being attached to the unitary assembly, including the pair of electrodes and resistance materials, by screws 5|.

In view of the foregoing, it will be seen that I have provided an improved protective device which, is simple in construction, economical to manufacture and efficient in operation. Furthermore, the shape of the exposed electrode is such as to substantially enclose the conducting members which may attain a relatively high voltage, the exposed conducting surface being adapted at all times to be connected to ground potential.

On sheet 2 of the drawings I have illustrated a modified protective device which is relatively rugged in construction and which has a plurality of resistance disks so that it may be employed to protect an electric apparatus of relatively high capacity. The construction includes, as will be seen in Figs. 5 and 6, a pair of resistance members and SI which may be made of any suitable material such as the same material mentioned above in connection with disk M. A first pair of electrodes 62 and 63 are provided on either side of the disk 60 and a second pair of lectrodes 64 and 65 are provided on either side of the second disk 6|.

In order to provide an arrangement for short circuiting the resistance disks 60 and BI so as to prevent them from becoming overheated during operation of the device, I may provide any suitable thermal responsive device or control device which is operative in response to the current flowing through the resistance disk or in response to a function of the temperature of the resistance disk. In the construction as is illustrated in Fig. 6 the thermal responsive device includes a bimetallic member 66, one end of which is supported through a conductor 61. So as to provide a convenient way of spacing the two pairs of disks and cooperating electrodes as well as mounting the thermal responsive device I provide a member 68 of suitable material such as a molded insulated product which has a cavity 69, as will be seen more clearly in the exploded view of Fig. 7 in which the thermal responsive device is supported.

One end of the conducting member 61, as will be seen in Fig. 6, is supported on the wall 10 of the insulating member through a rivet 1 I Upon the outer end of the bimetallic member 66 is carried a contact 12 which cooperates with a relatively stationary contact 13 which is supported on the wall member 14 0f the insulating spacer 68.

In order to electrically connect the protective device including the resistance plates across a suitable electrical apparatus to be protected, such as the secondary of a current transformer, there is provided a relatively high voltage terminal 15 and a relatively low voltage terminal 16. As will be seen in the diagrammatic representation in Fig. 8, the terminal 15 is adapted to be connected to the secondary of a current transformer 11 through a conductor 18 while the terminal 16 is adapted to be connected to the opposite end of the current transformer secondaries through a conductor 19. It is to be understood that the terminal 19 may be grounded if desired as is indicated at 80. The terminal 15 is in the form of a. hollow conductor through which extends a wire 8 I, the inner end of which is soldered to the conducting strap 82 as illustrated particularly in Fig. '7, the outer end being soldered to the outer end of the bushing as is illustrated at 83.

In order to provide a construction which is relatively shock proof and which also has a casing Furthermore, the

spun over as is indicated at and contact the adjacent electrode 02 so as to relatively tightly support the various parts of the protective device inside the enclosing casing including the lower electrode 55, the integral side wall and the upper electrode 02. A metallic plate member 80 may be provided which is attached to the lower elec trode in any suitable manner such as by welding so as to provide a relatively rugged supporting foot for the electrical device.

In order to provide an arrangement for insulating the relatively high voltage lead from the surrounding casing 05 I provide an opening Si in the casing through which extends an insulating member 02 which in turn has an opening through which extends the conducting member I5. Integral with the insulating member 92 is provided an insulating plate member 93 for cooperating with the inner face of the wall 05, and a suitable washer 94 of insulating material is provided which surrounds the insulating member 92 and prevents the washers and nuts 95 between which the lead from the wire I0 may be attached, from touching the casing 05.

So as to provide an arrangement for heating the bimetallic strip in response to or as a function of the current or temperature of the resistance disks, I provide a heater I00 adjacent the bimetallic disk which has one end IOI electrically connected to the conducting strap 51 which in turn connects with the high voltage lead I5. The other end of the heater I00 connects with a rivet I02 which will be seen in Fig. 5, and in the detail oi Fig. 9. The rivet is in turn supported on a wall I03 of the insulating spacer 58 with a pair of spring members I04 and I05 supported on opposite sides of the wall I00 and electrically connected to the rivet I02. It will be seen that as the parts are assembled in the manner shown in Fig. 6, the spring I04 will contact the adjacent electrode 53 while the spring I05 will contact the adjacent electrode 54. connection may be traced from the high voltage terminal I5 through the conductor 0| to the bimetallic element 05 and through the heater 80 and springs I04, I05 to the inner electrodes 63 and 04.

In order to provide a relatively low voltage connection between the terminal I0 and the outer shell to the contact II, a screw I00 is provided which contacts with the bottom of the electrode 05, as will be seen in Fig. 2, and screws into a nipple of suitable conducting material IIO on the end of which is supported a conducting strap III, the lower end of the strap I II being in turn connected to the contact I3. Since the integral side walls 00 connect through the spun-over ends 01 to the upper electrode 52, it will be seen that both of these electrodes are connected to the terminal which in turn connects with the low voltage side of the secondary winding through the conductor I0. It will be seen that the disk 5| has an internal base III and the electrode above I I 5 through which the member I I0 extends so that there is no electrical connection.

Operation of my improved protective device is shown in Figs. 5 through 9 and is as follows: With the protective device connected across the secondary winding 11 of a current transformer which is in turn connected to an ammeter as indicated by the numeral H5 in Fig. 8, so long as normal voltage is impressed across terminals I5 and I6 of the protective device substantially no current will flow through the protective device. However, when a relatively high voltage is im- Thus the high voltage pressed across the terminals occasioned, as by an open circuit in the secondary circuit, current flow may be traced in the diagrammatic view of Fig. 8 through the high voltage terminal I5, the conductor BI, and through the heater I00 to the conducting straps I04 and I05. The straps are, of course, connected to the electrodes 53 and 04. Current may then flow through the resistor disks 60 and BI to the electrodes 62 and 65. The circuit may then be traced out the low voltage terminal I6 and from the conducting electrode 19 to the opposite side of the secondary winding and also to ground through member 00. It will, of course, be understood that the heater device may be calibrated in relation to the thermostatic device such that the contacts I2 and I3 will be made to close when the resistor disks 60 and BI have obtained any suitable temperature. Thus it may be said that the thermal responsive device is operative in response to the function of the temperature of the resistor disks. Upon closure of the contacts I2 and I3 the disks will be short circuited so that the current may flow, as will be seen in Fig. 8, from the conducting strap 8| through the bimetallic strip 8! through the contacts I2 and I3 and directly to the low voltage terminal 16. With the current removed from the heater element I00 it will be understood that after a suitable time, depending upon the characteristics of the device, the contacts will again open thus causing current to again flow through the resistor disks. This operation will, of course, continue until the operator recloses the secondary of the transformer through the meters and leads.

In the construction as is shown in Figs. 5 through 9 it will be seen that I have provided a device which has an enclosing housin which is not only adapted to be connected to ground but I have also provided a relatively rugged construction which may be considered shock proof. It is to be understood that the high voltage terminal I5 may be turned so that it projects away from the normal position of the operator, or if desired an insulating cap member II2 may be provided over the bushing so as to protect the operator from coming in contact with this high voltage lead, the cap, of course, having an opening through which the insulated conductor I9 may project.

Modifications of the particular arrangements which I have disclosed embodyin my invention will occur to those skilled in the art, so I do not desire my invention to be limited to the particular arrangements set forth and I intend in the appended claims to cover all modifications which do not depart from the spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A protective device including a pair of electrodes adapted to be connected to an electrical apparatus to be protected, a resistance material connected between said electrodes, a temperature responsive device thermally associated with said resistance material, and a switch havin a pair of contacts connected across said electrodes. said temperature responsive device bein adapted to actuate said contacts upon being heated by heat from said resistance material, one of said electrodes being cup shaped so as to substantially enclose said other electrode and said resistance material.

2. A protective device including a pair of electrodes adapted to be connected to an electrical apparatus to be protected, a resistance material connected between said electrodes, a temperature responsive device thermally associated with said resistance material, a switch having a pair of contacts connected across said electrodes, said temperature responsive device being adapted to actuate said contacts upon being heated by heat from said resistance material, one of said electrodes being cup shaped so as to substantially enclose said other electrode and said resistance material, and means for grounding said cup shaped electrode.

3. A protective device including a pair of electrodes having substantially fiat parallel adjacent surfaces, a plate of resistance material held between said adjacent surfaces of said electrodes, a temperature responsive device thermally associated with said resistance material, a switch having a pair of contacts connected across said electrodes, said temperature device being adapted to actuate said contacts upon being heated by heat from said resistance material, one of said electrodes being cup shaped so as to substantially enclose said other electrode and said resistance material, said cup shaped electrode being adapted to be grounded. I

4. A protective device including a'pair of electrodes adapted to be connected to an electrical apparatus to be protected, said electrodes having adjacent faces with substantially fiat parallel portions, a resistance material between said adjacent faces of said electrodes, one of said electrodes having a recessed portion, a temperature responsive device within said recessed portion so that it will be afiected by heat from said resistance material, a switch having a pair of contacts connected across said electrodes, said temperature responsive device being adapted to close said contacts upon said resistance material becoming over-heated so as to short circuit said resistance material.

5. A protective device including a pair of elec trodes having adjacent faces with substantially fiat parallel portions, a plate of resistance material held between said adjacent faces of said electrodes, a switch having contacts connected across said electrodes, a temperature responsive device being adapted to actuate said switch upon said temperature responsive device becoming heated, one of said electrodes having a recessed portion defining a chamber facing said resistance material, and means for supporting said temperature responsive device within said chamber so that it will be affected by heat from said resistance material. a

6. A protective device including a pair of electrodes having adjacent faces with substantially flat parallel portions, a plate of resistance mate rial held between said adjacent faces of said electrodes, a temperature responsive device being adapted to actuate said switch upon said temperature responsive device becoming heated, one of said electrodes having a recessed portion defining a chamber facing said resistance material, and means for supporting said temperature responsive device within said chamber so that it will be affected by heat from said resistance material, said other electrode being cup shaped so as to substantially enclose said one electrode and in substantial registry, a terminal extending into said bores and being electrically connected to said second electrode, means for insulating said terminal from said plate of resistance material andsaid first electrode, a temperature responsive devic thermally associated with said resistance material and having a pair of contacts adapted to be actuated upon said temperature device becoming heated, and means for connecting one of said contacts to said terminal and said other of said contacts to said first electrode so that said electrodes will be short circuited upon said contacts being closed by said temperature responsive device.

8. A protective device including a first and a second electrode having adjacent faces with substantially fiat parallel portions, a plate of resistance material held between said adjacent faces of said electrodes, said first electrode and said plate of resistance material having bores in substantial registry, a terminal extending into said boresand being electrically connected to said second electrode, means for insulating said terminal from said plate of resistance material and said first electrode, a temperature responsive de vice having a pair of contacts adapted to be actuated by said temperature responsive device, said first electrode having a recessed portion defining a chamber facing said resistance material, means for supporting said temperature responsive device within said chamber so that it will be affected by heat from said resistance material and for connecting one of said contacts to said first electrode, and means including said terminal for connecting said other contact to said second electrode.

9. A protective device including a first and a second electrode having adjacent faces with substantially flat parallel portions, a plate of resistance material held between said adjacent faces of said electrodes, said first electrode and said plate of resistance material having bores in substantial registry, a terminal extending into said bores and being electrically connected to said second electrode, means for insulating said terminal from said plate of resistance material and said first electrode, a temperature responsive device having a pair of contacts adapted to be actuated upon said temperature responsive device becoming heated, said first electrode having a" recessed portion defining a chamber facing said resistance material, means for supporting said temperature responsive device within said chamber so that it will be affected by heat from said resistance material and for connecting one of said contacts to said first electrode, means including conducting and insulating portions for supporting and insulating said other contact on said first electrode, and a conductor for connecting said terminal with said conducting portion of said last mentioned means.

10. A protective device including a first and a second electrode having adjacent faces with substantially flat parallel portions, a plate of resistancev material held between said adjacent faces of said electrodes, said first electrode and said plate of resistance material having bores in substantial registry, a terminal extending into said bores and being electrically connected to said second electrode, means for insulating said terminal from said plate of resistance material and said first electrode, a temperature responsive device having a pair of contacts adapted to be actuated upon said temperature responsive device hecoming heated, said first electrode having a recessed portion defining a chamber facing said resistance material, means ior supporting said temperature responsive device within said chamber so that it will be aiiected by heat from said resistance material and ior connecting one oi said contacts to said first electrode, and means including said terminal ior connecting said other contact to said second electrode, said second electrode being cup shaped so as to substantially enclose said first electrode and said resistance material, said second electrode being adapted to be grounded.

11 A protective device including a pair of electrodes adapted to be connected to an electrical apparatus to be protected, a resistance material connected between said electrodes, a temperature responsive device having contacts connected to said electrodes so that said resistance material may be short circuited by the closing oi said contacts, said temperature responsive device being adapted to actuate said contacts in response to current flow through said resistance material, one oi said electrodes being cup shaped so as to substantially enclose said other electrode and said resistance material.

12. A protective device including a pair of electrodes adapted to be connected to an electrical apparatus to be protected, a disc-shaped resistance material connected between said'electrodes,

a temperature responsive device having contacts a connected to said electrodes so that said resistance material may be short circuited by the closing oi said contacts, said temperature responsive device being adapted to actuate said contacts in response to the heating eflect oi current flow through said resistance material, and means ior grounding one oi said electrodes.

13. A protective device including a pair oi electrodes having substantially flat parallel adjacent surfaces, a plate of resistance material between said adjacent suriaces oi said electrodes,

. a temperature responsive device adjacent one oi said electrodes, said temperature responsive device having a pair oi contacts connected to said electrode and operative upon being heated as a iunction oi the temperature of said resistance plate to short circuit said plate, one oi said electrodes being cup shaped so as to substantialliy enclose said other electrode and said resistance plate.

14. A protective device including pair oi electrodes adapted to be connected to an electrical apparatus to be protected, a resistance material connectedbetween said electrodes, 2. temperature responsive device having a pair oi contacts connected to said electrodes ior short circuiting said resistance material upon closure oi said contacts, heater means adjacent said temperature responsive device, means ior connecting said heater device to said electrodes so that said temperature responsive device will be heated a a iunction oi the current flow through said resistance material, one oi said electrodes being cup shaped so as to substantially enclose said other electrode and said resistance material.-

15. A protective device including two pairs oi spaced electrodes, resistance material connected between each of said pairs oi electrodes, 9, thermal responsive device having a pair oi contacts, means including insulating means ior supporting said thermal responsive device between one electrode oi one pair and another electrode oi said other pair, means for connecting one electrode oi each pair and the other electrode of each pair to an electrical apparatus to be protected, means ior connecting said contacts to said electrodes so that closing oi said contacts will short circuit said resistance material, means for heating said thermal responsive device as a iunction oi the temperature oi said resistance material during operation of said device, and means including one oi said electrodes of one pair and another oi said electrodes of the other oi said pairs acting as an enclosing housing ior. the other parts oi said device.

16. A protective device including a first pair oi electrodes, a plate of resistance material connected between said electrodes, a second pair oi electrodes, a second plate oi resistance material connected between said second pair oi electrodes, a thermal responsive device having a pair -'oi contacts, mean including insulating means ior supporting said thermal responsive device between said first and said second pair oi elec trodes, heater means adjacent said thermal responsive device, a pair oi terminals, means for connecting one oi the electrodes oi each oi said pairs and one contact to one terminal, means ior connecting the other oi said electrodes oi each of said pairs and said other contact to said other terminal, and means ior operating said thermal responsive device in response to a iunction oi the temperature oi said resistance plates during operation of said device.

17. A protective device including a first pair oi electrodes, a plate 0! resistance material connected between said electrodes, a second pair oi electrodes, a second plate oi resistance material connected betweensaid second pair of electrodes, a thermal responsive device having a pair oi contacts, means including lnsulatingmeans for supporting said thermal responsive device between said first and said second pair oi electrodes, heater means adjacent said thermal responsive device, a pair oi terminals, means ior connecting one oi the electrodes oi each of said pairs and one contact to one terminal, means ior connecting the other oi said electrodes of each of said pairs and said other contact to said other terminal, and heater means connected with said electrodes o that said thermal responsive device may be heated as a function of the temperature oi said resistance disks during operation of said device.

18. A protective device including a first pair oi electrodes, a plate oi resistance material connected between said electrodes, a second pair oi electrodes, a second plate oi resistance material connected between said second pair oi electrodes, a thermal responsive device having a pair oi contacts, means including insulating means ior supporting said thermal responsive device between said first and said second pair of electrodes, heater mean adjacent said thermal responsive device, a pair oi terminals, means ior connecting one oi the electrodes oi each of said pairs and one contact to one terminal, means ior connecting the other oi said electrodes oi each of said pairs and said other contact to said other closing housing for the remaining parts of said device.

19. A protective device including a plurality of spaced plates of resistance material with plate electrodes on opposite sides thereof, insulating means spacing adjacent electrodes of adjacent plates of resistance material, control means supported by said insulating means for short circuiting said resistance plates in response to overload conditions, one of said electrodes having an integral side providing a tubular shaped member, said side having an integral lip cooperating with another of said electrodes providing a relatively tightly assemblage of parts within said enclosing electrodes. I

20. A protective device including a pair of electrodes adapted to be connected to an electrical 

